1. Field of the Invention
This invention relates to a crystal growth method of oxide, cerium oxides, promethium oxides, multi-layered structure of oxides, manufacturing method of field effect transistor, field effect transistors, manufacturing method of ferroelectrics non-volatile memory and ferroelectric non-volatile memory, which are particularly suitable for use to oxide electronics developed on silicon substrates.
2. Description of the Related Art
Silicon oxide (SiO2) films made by thermal oxidation of silicon (Si) have been exclusively used as gate insulating films of MOS-FET (metal-oxide-semiconductor FET) because of their high electric insulating ability, low interface state density, easiness to process, thermal stability, and other advantages. These SiO2 films made by thermal oxidation for use as gate insulating films, however, have low specific dielectric constants (εr-3.8) and must be formed very thin on Si substrates. Along with the progress toward thinner gate insulating films, short channels and other requirements to meet the demand for integration, various problems arose such as dielectric break-down of gate insulating films and pinch-off of channels caused by influences from source-drain voltages (short channel effect), and gate insulating films will soon come to limit in terms of their materials. Under the circumstances, the need for new gate insulating films having high dielectric constants is being advocated as a technical subject of MOS-FET of the sub 0.1 micron generation, in addition to a further progress of lithographic technologies, needless to say (for example, (1) MTL VLSI Seminar (Massachusetts Institute of Technology)). If a gate insulating film can be made by using a material of a high dielectric constant, it will need not be so thin. Therefore, gate leakage will be prevented, and short-channel effects will be prevented as well.
On the other hand, researches on ferroelectric non-volatile memories (FeRAM) have come to be active (for example, (2) Appl. Phys. Lett., 48(1986)1439, (3) IEDM Tech. Dig., (1987)850, (4) IEEE J. Solid State Circuits, 23(1988)1171, (5) 1988 IEEE Int. Solid-State Circuits Conf. (ISSCC88), (6) Digest of Technical Papers, THAM 10.6 (1988)130, and (7) Oyo Butsuri, 62(1993)1212). Among these ferroelectric non-volatile memories, what is considered to be closest to practical use is a ferroelectric non-volatile memory of a quasi-DRAM structure (using two-transistors and two-capacitors type memory cells, or one-transistor and one-capacitor type memory cells). This structure is advantageous in making it easier to prevent interference with the Si process because CMOS process and ferroelectric capacitor process can be separated by using an inter-layer insulating film. However, the structure of this ferroelectric non-volatile memory does not meet the use of a scaling law of a Si device. Therefore, as microminiaturization progresses, it is necessary to employ a more complex structure or use a material with a larger value of residual polarization in order to ensure a certain amount of charge storage in the capacitor. On the other hand, there are many research institutes tackling with the study of ferroelectric non-volatile memories using MFS-FETs (metal-ferroelectrics-semiconductor)-FET type memory cells, including MFMIS (metal-ferroelectrics-metal-insulator-semiconductor)-FET type memory cells, FCG (ferroelectric capacitor gate) type memory cells, and so forth), which constitute two major subjects together with those of a quasi-DRAM structure mentioned above. The latter type ferroelectric non-volatile memories match with the scaling low, and merely need a very small value of residual polarization (about ˜0.1 μC/cm2). Additionally, since they need only one transistor for storage and hence contribute to a decrease of the cell size, they are advantageous for high integration. Furthermore, since they are of a nondestructive readout type, they are more advantageous also against fatigue, which might be an essential problem of ferroelectric materials, than destructive readout type memory cells with two transistors and two capacitors, or one transistor and one capacitor, and are also available for high-speed operations. Because these excellent properties are expected, MFS-FET ferroelectric non-volatile memories are now recognized as ultimate memories ((8) Appl. Surf. Sci. 113/114(1997)656).
It is problems with their manufacturing process that prevent practical use of these MFS-FET ferroelectric non-volatile memories. It is extremely difficult to grow ferroelectric materials directly on Si substrates. Therefore, growth of buffer layers of insulating materials on Si substrates is recognized as one of most important technologies. In case of a MFIS (metal-ferroelectrics-insulator-semiconductor)-FET ferroelectric non-volatile memories which is one of MFS-FET ferroelectric non-volatile memories, gate voltage is distributed to an insulating layer as well, and this causes the drawback that the write voltage is high. To prevent it, the insulating layer must be one with a high dielectric constant. On the other hand, material properties required as a ferroelectric material used here are a low dielectric constant, appropriate value of residual polarization (typically around ˜0.1 μC/cm2, although depending upon the device design), and most seriously, good squareness ratio. Additionally, for the purpose of realizing a better interface, it is the important condition that these materials can be grown at low temperatures. Thus, choice and development of materials is required from the standpoint different from that of the one-transistor and one-capacitor type. There are a lot of research reports on MFIS-FET ferroelectric non-volatile memories. However, because of insufficient surface properties, there is almost no reports about practically usable ones including the requirement for retention (charge retaining property). On the other hand, a MFMIS structure enabling the use of an existing SiO2 film by thermal oxidation as the gate insulating film is also under consideration ((9) Jpn. J. Appl. Phys., 33(1994)5207), and this is considered to be relatively close to the stage of practical use. There is also proposed an approach stepping forward from that by separately making a ferroelectric capacitor and connecting it to a polycrystalline Si gate by wiring ((10) Japanese Patent Laid-Open Publication No. hei 8-250608 and (11) Japanese Patent Laid-Open Publication No. hei 9-205181). This method facilitates device isolation between a ferroelectric material and a Si transistor, and at the same time, because the design choice in areal ratio between the capacitor and the gate, sufficient polarization can be obtained with a low write voltage by reducing the relative area of the capacitor. However, it is difficult to reliably obtain a necessary squareness ratio with polycrystalline ferroelectric materials, and this method will also encounter the limit of the material of the SiO2 film by thermal oxidation in progress of microminiaturization. Eventually, also the key technology for realizing MFS-FET ferroelectric non-volatile memories is just the growth of an insulating film with a high dielectric constant on a Si substrate. Moreover, in order to realize a steep interface and a low interface state density equivalent to SiO2 films by thermal oxidation, it is advantageous to epitaxially grow a material good in lattice matching, and there is such an extremely high technical hurdle that a channel can be made in the Si<110>orientation having the largest mobility of Si as the substrate and it should be on a Si(001) substrate being used exclusively as the MOS-FET substrate.
On the other hand, it is greatly significant to introduce oxide materials other than SiO2 into the semiconductor industry. High-temperature superconductive materials discovered in 1986 ((12) Z. Phys. B., 64, 189–193(1986)), needless to say, and oxide materials especially having perovskite or related structures have very important physical properties for semiconductor devices, such as ferroelectricity, high dielectric constant, superconductivity, colossal magnetoresistance, and so forth ((13) Mater. Sci. Eng., B41(1996)166, and (14) J. Ceram. Soc. Japan, Int. Ed., 103(1995)1088). For example, among ferroelectric materials of capacitors for ferroelectric non-Volatile memories mentioned above, zirconium titanate (PZT) having a large value of spontaneous polarization and a low process temperature (for example, (15) J. Appl. Phys. 70, 382–388(1991)) and bismuth strontium tantalate (Bi2SrTa2O9 ((16) Nature, 374(1995)627, (17) Appl. Phys. Lett., 66(1995)221, (18) Mater. Sci. Eng., B32(1995)75, (19) Mater. Sci. Eng., B32(1995)83, (20) Appl. Phys. Lett., 67(1995)572, (21) J. Appl. Phys., 78(1995)5073, (22) Appl. Phys. Lett., 68-(1996)566, (23) Appl. Phys. Lett., 68(1996)690, and (24) International Laid-Open Publication WO93/12542) are the twin greatest materials. Furthermore, including the discovery of colossal magnetoresistance materials (CMR materials) in the group of Mn oxides, which are variable in resistivity over some digits under application of a magnetic field ((25) Phys. Rev. Lett. 74(1995)5108), great interest has come to be attracted to how high potential capacities these oxide materials have ((26) Mater. Sci. Eng., B41(1996)166, and (27) J. Ceram. Soc. Japan, Int. Ed., 103(1995)1088), and technologies for making thin oxide films have been developed remarkably in these ten years or so.
If oxide materials having these very high functional physical properties can be developed on Si which is the basis of the semiconductor industry, these materials will get a high marketability. However, because of difficulties between these functional oxide materials and Si, such as mutual thermal diffusion and differences in thermal expansion coefficient, it is usually difficult to directly grow these functional oxide materials on Si.
As discussed above, almost all of these functional oxide materials have structure based on a perovskite structure. Many of them, such as yttrium-based superconductive materials having critical temperatures beyond the liquid nitrogen temperature and Bi2SrTa2O9 mentioned above, have structures called layered perovskite having a very large anisotropy. In these layered perovskite structured oxides, superconductive current paths, polarization axes, etc. are limited to specific directions, and also in case of simple perovskite structured oxides, there are many in which polarization axes are limited to specific directions like that in PZT. Therefore, when they are used to make devices, it is important to specifically orient oxides, or more preferably, epitaxially grow them relative to bases, in order to draw out their maximum properties.
Ceria (cerium dioxide: CeO2) having a fluorite structure is one of candidates for materials of gate insulating films with high dielectric constants for the sub 0.1 micron generation to substitute for SiO2 films by thermal oxidation because of its thermal stability, high specific dielectric constant (εr˜26) and very good lattice matching with Si substrates (misfit: about 0.35%), and it is considered to be one of most ideal buffer layer materials for epitaxially growing perovskite-related oxides on Si substrates. Actually, researches are being made on epitaxial growth of ceria on Si substrates, but most of them are directed to CeO2(111)/Si(111) structures easy for atomic close-packed growth (for example, (28) Jpn. J. Appl. Phys. 34(1995), L688, and (29) Japanese Patent Laid-Open Publication No. hei 7-25698). However, with regard to Si(001) substrates which are most important for practical application, there is the story that, under the recognition that the epitaxial CeO2(001)/Si(001) structure was just the ideal structure, various proposal were made on multi-layered structures, devices, and so on ((30) Japanese Patent Laid-Open Publication No. hei 2-267104, (31) Japanese Patent Laid-Open Publication No. hei 6-97452, (32) Japanese Patent Laid-Open Publication No. hei 10-182292, and others), but it was not realized. Heretofore, it has been believed that CeO2(110) having an antiphase domain epitaxially grows, reflecting the dimer structure by the surface reconstructed structure of Si(001)-2×1, 1×2. Further, regarding the growth temperature, it is considered that a temperature around 800° C. at which no SiO2 film is formed on the Si surface in a high vacuum ((33) J. Vac. Sci. Technol. A13(1995)772) is the lower limit of epitaxial temperature ((34) Jpn. J. Appl. Phys. 33(1994), 5219, (35) Appl. Phys. Lett. 56 (1990), 1332, (36) Appl. Phys. Lett. 59(1991), 3604, (37) Physica C 192 (1992)154, (38) Jpn. J. Appl. Phys. 36(1997), 5253, and (39) Japanese Patent Laid-Open Publication No. hei 9-64206). Furthermore, under the belief that the growth temperature must be lowered to obtain a steep interface, electron beam assisted epitaxial growth, for example, is under trial. However, the lower limit of epitaxial growth temperature heretofore reported is 710° C. ((40) 1998 spring symposium of Oyo Butsuri Gakkai, Presentation No. 28p-PA-1). Although there are a few reports on CeO2(001)/Si(001) structures, there is no experimental data ((41) Japanese Patent Laid-Open Publication No. hei 2-267104), no discussion is made about separation from substrate diffraction ((42) Solid State Comm. 108(1998)225), and none could prove epitaxial growth. Although there are proposals of solid solution (Ce, Zr)O2 of zirconia (zirconium dioxide: ZrO2) and ceria ((43) Jpn. J. Appl. Phys. 35(1996), 5150), CeO2/(Ce, Zr)O2/Si multi-layered structure ((44) Jpn. J. Appl. Phys. 36(1997), 5253 and (45) 1998 spring symposium of Oyo Buturi Gakkai, Paper 29p-ZF-4), and CeO2/SrTiO3/Si structure ((46) Jpn. J. Appl. Phys. 30(1991)L1136), which use the solid solution as a buffer layer. However, it has not been reported at all that CeO2(001) could be epitaxially grown directly on a Si(001) substrate.
Researches are being made also on yttria (yttrium oxide: Y2O3) having a C-rare-earth structure (bixbyite) because it has material properties similar to ceria. However, it involves the same problems as ceria, and in usual, Y2O3(110) epitaxially grows on Si(001) substrates ((47) Appl. Phys. Lett. 71(1997), 903).
On the other hand, there is an example in which a perovskite oxide epitaxially grows on ceria (or yttria) (for example, (48) Appl. Phys. Lett. 68(1996)553). Therefore, if ceria can be controlled in orientation, a device making use of the characteristics of functional oxides will be realized.
Under these circumstances, technologies for epitaxially growing CeO2(001) on Si(001) substrates most important for practical application are greatly important.